[GRASS-user] Re: i.atcorr negative DEM

[Elena Mezzini]

Markus Metz-2 wrote:
> 
> On Tue, May 17, 2011 at 10:08 PM, Elena Mezzini
> <elena.mezzini at gmail.com> wrote:
>>
>> On Tue, May 17, 2011 at 9:22 PM, Elena Mezzini
>> <elena.mezzini at gmail.com> wrote:
>>> Hi all,
>>> I'm once again in trouble with i.atcorr in GRASS6.5.svn.
>>> I notice that my output reflectance images are strange: there are some
>>> areas
>>> that are really bright and other that are really dark. Both seem to
>>> match
>>> with my DEM raster.
>>> Could there be problems with negative values in DEM?? I'm in a coastal
>>> area
>>> and there are some real altitude negative values.
>>
>> i.atcorr (the 6S code it is using to be exact) should treat negative
>> elevation values as elevation = 0. It is debatable if this is correct,
>> but in theory i.atcorr should behave accordingly. Actually, I am not
>> sure about it's behaviour for negative elevation values...
>>
>> You could set all negative elevation values to zero,
>>
>>
>> Ok, how can I do it?
>>
> replace my_elevation with the name of your elevation map:
> 
> g.region rast=my_elevation
> r.mapcalc "elevation_positive = if(my_elevation < 0, 0, <my_elevation>)"
> 
> 
> 
> g.region rast=dem_sross_srtm
> r.mapcalc "dem_sross_srtm_pos = if(dem_sross_srtm < 0, 0,
> <dem_sross_srtm>)"
> 
> Syntax error!
> As you know I'm not a programmer... where is the error?
> 
> Thank you once again
> 
> Elena
> 

Found:

g.region rast=dem_sross_srtm
r.mapcalc "dem_sross_srtm_pos = if(dem_sross_srtm < 0, 0, dem_sross_srtm)"

Sorry!

Elena


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